Not all body fat is equal: how exercise makes fat tissue healthier

By Gretchen Reynolds

18 December 2017 — 10:59am

Exercise could help to make your fat tissue healthier, which is a good thing. According to a timely new study, a single session of exercise may change the molecular workings of fat tissue in ways that, over time, should improve metabolic health.

This finding has particular relevance during the holidays, when, despite our best intentions, so many of us add to our fat stores. Exercise might make these annual bacchanals less metabolically damaging than otherwise.

We tend to add to our fat stores during the holiday season, but exercising can minimise the damage of overindulging.

Photo: Studio Firma

Most of us probably think of our fat tissue as inert and undesirable. But our fat is, in fact, a busy and necessary tissue, producing and sending out multiple biochemical signals that affect biological operations throughout the body.

Fat tissue's most important responsibility is to securely store fat, and we should hope that it performs this function well. Provocative recent research in both animals and people has found that, if a person's or animal's fat tissue is relatively leaky, allowing fatty acids to ooze into the bloodstream, roving fat blobs can accumulate in other tissues, particularly the muscles and liver. Once there, they contribute to the development of insulin resistance, a serious metabolic condition that often leads to diabetes.

In a study published this year, for instance, scientists from the University of Michigan and elsewhere found that if overweight men and women had low levels of fatty acids in their bloodstream, they also were metabolically healthier than other overweight adults.

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Even more interesting, they generally also had healthy fat, the scientists found, with biopsies showing less inflammation and scarring than in the fat from other overweight men and women. (This fat was subcutaneous, meaning it came from just beneath the skin.)

Presumably, the scientists speculated, this robust fat was leaking less than the frailer variety.

But that study did not examine why some people had healthier fat than others and whether the condition of anyone's fat tissue might be changed.

So for the new study, which was published last month in the Journal of Applied Physiology, the same group of scientists began to consider exercise.

Exercise, of course, is well known to affect the amount of fat we store, since muscles use fatty acids as fuel. Exercise also is believed to prompt small amounts of white fat to transform into brown fat, a particularly desirable form of fat that burns a lot of calories.

But it has not been clear whether exercise directly alters the health of white fat tissue.

To find out, the researchers first gathered 20 men and women who were overweight but did not have insulin resistance. Eight of them exercised regularly. The others had been sedentary.

The researchers tested their volunteers' body compositions and took fat samples. Then they had each volunteer exercise on a treadmill or stationary bike for an hour at a moderately tiring pace.

An hour later, the scientists repeated the fat biopsies.

Examining the various tissues microscopically afterward, the researchers found several tantalizing differences.

In almost all of the volunteers, the fat tissue after exercise showed greater amounts of a protein that is known to contribute to the development of more blood vessels.

This change could be important over time, said Jeffrey Horowitz, a professor of movement science at the University of Michigan School of Kinesiology who conducted the experiment with Douglas Van Pelt (now a postdoctoral researcher at the University of Kentucky) and others.

Interestingly, the fat tissue from those volunteers who regularly exercised also showed a small but meaningful increase in genetic activity related to blood vessel proliferation, suggesting that their tissue was more primed than that from the sedentary volunteers to create additional blood flow.

Their fat tissue also showed a slight increase in the gene expression of a substance that helps to reduce inflammation.

These alterations were not enormous, Horowitz said. But they occurred consistently and after a single session of exercise, he points out, and might, with continued exercise, improve fat health over time.

This study was small, however, and very short-term and did not look at whether other amounts or types of exercise would have comparable effects within fat. It also did not measure whether exercise actually changed the amounts of fat in the bloodstream and, since the volunteers were overweight, cannot tell us whether the effects would be comparable in people whose weight was normal.

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Perhaps most fundamentally, the study concentrated on how to make our fat's health rise when most of us would prefer that its quantity decline.

Horowitz understands: "There is no doubt that the best thing for metabolic health is to lose weight." But at this time of year, he said, when fat gain is common, a brisk walk or jog might make this added fat healthier and more stable, and the broader effects on our bodies a little less concerning.